Tissue, such as a benign or malignant tumor or blood clot within a skull or other region of a patient's body may be treated invasively by surgically removing the tissue, or non-invasively by using effects of focused ultrasound. Both approaches may effectively treat certain localized conditions within the brain, but involve delicate procedures in which it is desired to avoid destroying or damaging otherwise healthy tissue. These treatments may not be appropriate for conditions in which diseased tissue is integrated into healthy tissue, unless destroying the healthy tissue is unlikely to affect neurological function significantly.
The application of ultrasound energy, has been investigated as a potential primary and adjunctive treatment for thrombotic disease. High-intensity focused ultrasound (HIFU) has been shown to enhance thrombolysis induced by tissue plasminogen activator (tPA) in-vitro and in-vivo.
Focused ultrasound, has particular appeal for treating tissue within the brain because it generally does not disturb intervening or surrounding healthy tissue. Focused ultrasound may also be attractive because acoustic energy generally penetrates well through soft tissues and ultrasonic energy, in particular, may be focused toward focal zones having a cross-section of only a few millimeters due to relatively short wavelengths (e.g., as small as 1.5 millimeters (mm) in cross-section at one Megahertz (1 MHz)). Thus, ultrasonic energy may be focused at a small target in order to ablate the tissue without significantly damaging surrounding healthy tissue.
To focus ultrasonic energy toward a desired target, drive signals are sent to a piezoelectric transducer having a number of transducer elements such that constructive interference occurs at a “focal zone.” At the focal zone, sufficient acoustic energy may be delivered either to heat tissue until necrosis occurs, or mechanically disrupt its structure until the tissue is destroyed. Preferably, tissue along the path through which the acoustic energy passes (the “pass zone”) outside the focal zone is heated only minimally, if at all, thereby minimizing damage to tissue outside the focal zone.
Stroke is the third leading cause of death in the United States and a leading cause of adult disability. In general, strokes can be classified as either ischemic or hemorrhagic. In ischemic strokes, the blockage of blood flow results from a clot in intracerebral vessels, whereas hemorrhagic strokes are caused by ruptured blood vessels. Several clinical trials were initiated to evaluate the safety and efficacy of ultrasound-assisted tPA approach to treat stroke patients. However, these trials were generally unsuccessful due to adverse events associated with bleeding that were triggered outside the target area.
Accordingly, there is a need for systems and methods for effectively focusing acoustic energy to treat clots in a manner that does not adversely affect surrounding tissue and can be administered in a timely fashion.